1. Field of the Invention
The present invention relates to a copper alloy sliding material, more particularly to the copper alloy sliding material which is suitable to a sintering alloy bearing including a plain bearing.
2. Brief Description of the Art
In general, the Kelmet alloy (i.e. a Cuxe2x80x94Pb system or a Cuxe2x80x94Snxe2x80x94Pb system) is used for a copper alloy sliding material, for example a sintering copper alloy for plain bearings. It has been known that the Kelmet material has excellent anti-seizure property and exhibits good sliding property under a hydrodynamic lubrication condition by virtue of a much content of Pb (i.e. about 20 mass %). Recently, however, it has been desired for various metal materials not to contain Pb as far as possible from the view point of protecting the environment.
The present invention has been developed under the above background and is aimed to provide the copper alloy sliding material which can ensure a high performance of anti-seizure property while reducing the Pb content.
With regard to the copper alloy sliding material, there may be an idea of dispersing hard particles in the copper based matrix (e.g. Cuxe2x80x94Sn), the hard particles being much harder than the matrix. In general, the hard particles may be of a metal system material such as Mo or W, or of a ceramic system material such as SiO2, Al2O3 or SiC.
In this case, the following function or effects can be expected:
(1) Since the hard particles much harder than the copper based matrix are dispersed on the surface (i.e. sliding-contact surface), the copper alloy sliding material will have good sliding-contact property and excellent wear resistance.
(2) The hard particles will protrude from the surface so as to form recessions with relation to the matrix, so that the oil retaining property and anti-seizure property will be improved.
(3) The hard particles will make the surface of a mating shaft smooth to improve the anti-seizure property.
(4) Although there is a fear that the copper based matrix will partially move to the surface of the mating shaft (usually, steel) due to adhesion to deteriorate the anti-seizure property, the hard particles will shave off adhesives of the copper alloy from the mating shaft to contribute to improvement of the anti-seizure property and a long life of the mating shaft.
However, because the metal system hard particles consisting of Mo or W have a lower hardness (which is not more than 500 of Vickers Hardness) than the ceramic system hard particles, they are inferior in the effect of shaving off adhesives of the copper alloy from the mating shaft. There is also a problem that metals such as Mo or W are comparatively adhesive to steel (Fe) of the mating shaft as compared with ceramics because of the metal to metal sliding-contact. In contrast, the ceramic system hard particles such as SiO2, Al2O3 or SiC are excellent in the effect of shaving off adhesives of the copper alloy from the mating shaft because of higher hardness (for example, 600 to 700 of Vickers Hardness) than the mating shaft and do not adhere to the mating shaft because they are hard to form intermetallic compounds with metals such as steel (Fe).
However, in the case where the ceramic system hard particles such as SiO2, Al2O3 or SiC are selected, there have been a problem that, when the sliding contact material receives a dynamic load, since the protruded hard particles support the load, cracks arise at the interface between the copper system matrix and the hard particles due to shear stress arisen at the interface resulting in a phenomenon of falling-off of the hard particles from the copper system matrix. If such falling-off of the hard particles occurs, the abrasive wear arises resulting in deterioration of anti-seizure property. Further, when such falling-off of hard particles arises, voids (or recessions) are produced to become trigger points of fatigue due to stress concentration resulting in deteriorated fatigue resistance. The factor that hard particles such as SiO2, Al2O3 or SiC are liable to fall off from the matrix will reside in low wettability by copper (or a copper alloy).
Thus, the present inventors practiced various experiments and researches in order to solve the falling-off problem of the hard particles while ensuring the effect in virtue of the hard particles, whereby it has been confirmed that the wettability of the hard particles by the copper alloy matrix is improved to make them hard to leave from the matrix by using WC, W2C and/or Mo2C as the hard particles instead of usual SiO2, Al2O3 or SiC, consequently the present invention has been achieved.
The invention copper alloy sliding material comprises 0.5 to 15 mass % Sn and 0.1 to 10 vol % of hard particles consisting of one or more selected from WC, W2C and Mo2C. The content of the hard particles is defined not with the mass percent but with the volume percent, being more proper than the former unit which varies with the specific gravity of the hard particles, because a volume of the hard particles is important with regard to the function thereof. This is the same in the case of a solid lubricant mentioned below.
According to the copper alloy sliding material in which the hard particles with a high hardness are dispersed, excellent anti-seizure and fatigue resistance properties can be obtained while reducing the Pb amount. Since the hard particles consisting of one or more selected from WC, W2C and Mo2C have a high hardness (i.e. not less than 1300 of Vickers Hardness), they are excellent in the effect of shaving off adhesives of the copper alloy from the mating shaft and do not adhere to the mating shaft because they are hard to form intermetallic compounds with metals such as steel (Fe). Further, the hard particles consisting of one or more selected from WC, W2C and Mo2C have good wettability by the copper alloy contrasting with SiO2, Al2O3 or SiC, so that it is possible to prevent them to fall off from the copper alloy matrix.
The defined amount of Sn being 0.5 to 0.15 mass % can strengthen the copper alloy matrix to improve the fatigue resistance. If the Sn amount is less than 0.5 mass %, there can not be seen the effect of strengthening the copper alloy matrix. If the Sn amount exceeds 15 mass %, a much amount of Cuxe2x80x94Sn system intermetallic compounds is produced to become brittle disadvantageously.
The amount of the hard particles are required to be 0.1 to 10 volume %. If it is less than 0.1 volume %, the desired improvement in anti-seizure and wear resistance properties can not be obtained. If it exceeds 10 volume %, the strength is deteriorated to be lower fatigue resistance and an attacking intensity of the hard particles against the mating shaft becomes too great, so that there can not be seen the improvement effect of anti-seizure and wear resistance properties. More preferably, the amount of the hard particles is 0.5 to 5 volume %.
The hard particles have preferably an average particle size of 0.1 to 10 xcexcm. If the average particle size is less than 0.1 xcexcm, the hard particles are too fine, so that there can not be appeared improvement of anti-seizure and wear resistance properties as an elemental hard particle. If the average particle size exceeds 10 xcexcm, an attacking intensity of the hard particles against the mating shaft increases resulting in deterioration of workability including machinability. More preferably, the average particle size is within a range of 1 to 5 xcexcm.
The copper alloy may comprise an amount or a total amount of not more than 40 mass % of one or more selected from Ni, Ag, Fe, Al, Zn, Mn, Co, Si and P (phosphorous). The alloying element(s) strengthens the copper alloy matrix to improve the fatigue resistance of the copper alloy. If the amount of the alloying element(s) exceeds 40 mass %, the copper alloy matrix becomes too hard to apply the alloy material to bearings. Therefore, the amount or the total amount of the alloying element(s) is preferably not more than 40 mass %.
The copper alloy may also comprise an amount or a total amount of not more than 10 mass % of Bi and/or Pb. In this case, a soft Bi and/or Pb phase is formed in the copper alloy matrix to improve conformability, foreign substance embeddability and anti-seizure property of the copper alloy. If the amount of Bi and/or Pb exceeds 10 mass %, the strength of the copper alloy is deteriorated. Therefore, the amount of Bi and/or Pb is suitably not more than 10 mass %. With regard to Pb, while its amount is desirably as small as possible (for example, zero percent), it is possible to achieve the object of the invention, which is to fully reduce the Pb content in the copper alloy relative to the conventional the Kelmet alloy (comprising about 20 mass % Pb), by the defined amount of Pb.
The copper alloy may also comprise an amount or a total amount of not more than 10 vol % of a solid lubricant comprising BN, graphite, MOS2 and/or WS2. The solid lubricant can improve the self lubrication property of the copper alloy, so that the anti-seizure and wear resistance properties of the copper alloy can be further improved. If the amount of the solid lubricant exceeds 10 vol %, the strength of the copper alloy is deteriorated. Thus, the amount of the solid lubricant is properly of not more than 10 vol %.